Insects and the Gondwana Hypothesis

Before seeing how some insects provide support for the palaeogeological Gondwana Hypothesis, let us become familiar with the hypothesis. The hypothesis was suggested by Eduard Suess, an Austrian geologist. According to it, South America, Africa, Peninsular India, Australia and Antarctica were formed by breaking up of a large land mass or supercontinent in the southern hemisphere, and by subsequent drifting of the resulting land masses (continental drift). Suess gave the name Gondwanaland to the supercontinent of the south.

Even before Suess some elements of the hypothesis had been conceived by Alfred Wegener, a German geologist in 1912. He suggested that in the Palaeozoic era (about 255 million years back) there was a single huge land mass, which he called Pangea. About 245 million years ago it broke into two parts, a northern portion and a southern part, which subsequently moved away from each other. Later geologists named the northern continent Laurasia and the southern one Gondwanaland.

As per our present notion, by middle of the Jurassic period (about 160 million years ago) a huge sea, the Sea Tethys separated Laurasia in north from Gondwanaland in south. Later Laurasia broke up to form North America, Europe and Asia. Gondwanaland remained undivided up to Cretaceous (130 to 140 million years back), and then it fragmented to form South America, Africa, Madagascar, Peninsular India, Australia and Antarctica. Africa and Peninsular India became well separated from the southern land mass, and moved northward to become connected with the

Eurasian land mass. Madagascar, as an African satellite, remained in a splendid isolation. South America and Australia remained almost connected with Antarctica till about 75 million years back and, even later on, they maintained close proximity with the Antarctica.

The Gondwana Hypothesis, as outlined above, is well supported by geological evidence. Structure and stratigraphic sequences in Palaeozoic deposits match well in mountain ranges of eastern North America and northwestern Europe. Palaeozoic stratigraphic sequences are very similar in the southern continents and in the Peninsular India. In 1960s, shore lines on the two sides of the Atlantic at depth of 1000 m have been worked out through drilling and computer mapping, and they have been found to be quite fit for juxtaposition of the continents on the two sides, agreeing well with fragmentation of the old land masses, as suggested by the hypothesis.

The hypothesis receives convincing support from floral and faunal distribution too. Fossils of the seed fern Glossopteris are found in all the southern continents and in the Peninsular India. They are not met with anywhere else. McDaniel and Shaw (2003) have studied Australian and South American populations of the moss Pyrrhobryum mnioides using DNA analysis, and have inferred that the moss populations in the two geographical areas are monophyletic (i.e. having evolved from a common ancestral stock), and that there has been no intercontinental dispersal between them, which means the origin of the two sets of populations should be Gondwanian.

The Gondwana Hypothesis explains well the disjunct transpacific distribution as resulting from the sequential break-up of the supercontinent Gondwana, during the last 165 millions years. New Caledonia, New Guinea, New Zealand were still linked at the end of the Mesozoic (-65 myr) to what survived in the South of the great Gondwana continent, from Patagonia, via Antarctica and Australia. In that period, Antarctica was not tropical but it had a warm temperate climate, and that allowed migration of fauna and flora in both directions. During the mid-Cretaceous (-110 myr) South America and Africa began to drift away (Sanmartin, 2002; Sanmartin and Ronquist, 2004). Antarctica in the late Cretaceous (-65 myr) to early Eocene (-60 myr) was covered with an angiosperm-rich flora dominated by Nothofagus. Winteraceae, with Taktaja-nia in Madagascar, Monimiaceae with Hedycarya and Kibaropsis in New Caledonia, Amborellaceae, with Amborella in New-Caledonia, Aus-trobaileyaceae with Austrobaileya in Queensland are remains of that period. Many gymnosperms, like Araucaria and related (missing in Madagascar) are also all witness of that time. Several gymnosperms have a subantarctic-pacific distribution: Araucaria, Libocedrus, Prumnopitys and Retrophyllum. In New-Caledonia many plants are true relics of an old Cretaceous distribution (Richier de Forges et al, 1998). Among the angiosperms, Cunoniaceae, Proteaceae and many Myrtaceae also have had a Gondwanian origin. During most of the Paleogene (-70 myr), New Zealand and New Caledonia were progressively submerged, but some dry land remained and insect and plant relics survived there. It is probably in the Cretaceous that Nothofagus and Proteaceae colonized New Zealand and New Caledonia, from South America, following the antarctic northern margin of Gondwana (Richier de Forges et al, 1998). Another opinion, however, is that Nothofagus came from Australia to South America. An interesting genus, Gunnera (Gunneraceae), the terrestrial plant with the biggest leaves has also had a Gondwanian distribution (Wanntorp and Wanntorp, 2003), but is missing in New Caledonia and in continental Australia, though represented in Tasmania and New Guinea. It is of some interest, since Gunnera is somewhat related to Myrtaceae, which has been reported as harbouring occcasionally the genus Stenomela, one of our archaic Gondwanian eumolpines. No wonder why archaic eumolpines (Jerez, 1996) are found at the two extremities of the Southern Gondwana, in Chile and in New-Caledonia.

There are several interesting instances in animal distribution supporting the Gondwana Hypothesis. Lung fishes (constituting the natural group Subclass Dipnoi) show a Gondwanian distribution. Among them Cerato-dus is in Australia, Protopterus in Africa, and Lepidosiren in South America. The side-necked turtles (constituting the Suborder Pleurodira) are confined to the three southern continents. The flightless birds (constituting mostly the natural group Ratitae) are also almost confined to Australasia, Africa and South America. Among them ostriches are in North Africa, rhea in South America, emeus in Australia, cassowaries in New Guinea and Australia and kiwis in New Zealand. Some others are recently extinct (Moas, Sylviornis, Aepyornis), probably due to human interference, in New Zealand,

Australia and Madagascar. Marsupials (that is mammals giving birth to immature young ones, which have to be kept in a special pocket on the belly of the mother for allowing them to develop further, constituting the Subclass Metatheria) are in Australia and South America. Their presence in Central America and south part of North America is due to their northward dispersal, just as ostriches extend their range into the Arabian countries. Survival of marsupials only in these two southern continents has been attributed to their long isolation from other land masses in geological history, and consequent absence of competition with higher and more successful forms in these continents.

Insects also provide some instances supporting the hypothesis, but such cases are not many, mainly because insects are mostly small and light bodied and are provided with wings. These attributes of insects are responsible for their great migratory capacity and wide dispersal possibility (see the chapter "Insect Migrations"). One illustrative example of how successfully an insect may reach a new area and readily establish itself is that of the small yellow ants (Anoplolepis gracilipes). This ant was unknown in the Christmas Islands, which are close to Australia, till about 70 years ago. It is a carnivorous ant species, and is now a threat to the original life on the islands. It has voraciously eaten away small animals of all descriptions, including insects, crabs and birds. In absence of animal pollinators forest flora is being lost at a fast pace (Anonymous, 2004). It seems that the ant has entered the islands with floating timber, and, on finding absence of competition, abundance of food and favourable climatic conditions, has multiplied fast. A chrysomelid beetle, Chaetocnema confinis is, through typhoons and hurricanes, conquering the whole of the tropics from a North American origin, invading Africa, Asia and Oceania. Its success is due to parthenogenetic females, which, on reaching a new area, multiply by themselves. The beetle is bisexual in North America, probably its country of origin.

Some primitive genera of leaf beetles, belonging to the subfamily Sagrinae (Family Chrysomelidae), are almost confined to the Australian region (Australia and New Guinea), but the genus Megamerus occurs in Brazil and Madagascar, besides Australia, and the genus Sagra, more advanced, occurs in tropical Africa, India, Indonesia and China (Jolivet, 1997; Jolivet and Verma, 2002). Atalasis, closely related to Megamerus, occurs in Argentina. A

few fossil sagrines are known from the Northern hemisphere, but the living archaic genera are all from land masses of the southern Gondwanian origin. Sagrines went extinct in Europe and elsewhere. Eosagra from the Eocene of Germany seems to be a real sagrine, but others are doubtful. It should be noted that sagrines show a primitive way of feeding: they are borers into the stems or roots of young plants and their pupae are gallicolous (inducing formation of and living in plant galls). This brief account of life history holds for the sagrines, which have been studied. Practically nothing has been observed and recorded for the archaic sagrines of Australia, but they are expected to have a similar biology. There are no living sagrines in New-Zealand and New-Caledonia.

There are some primitive members of the subfamily Eumolpinae (Family Chrysomelidae) in Australia and South America, namely Spilopy-ra and Macrolema in Australia and New Guinea, Richmondia in Australia, Bohumiljania in New Caledonia, and Hornius and Stenomela in South America. It may be recalled that New Guinea and New Caledonia are in the Australian region, as is New-Zealand, which is devoid of these primitive eumolpines. Probably there they are extinct or they never reached the place. These primitive eumolpines have basic similarity among them (Verma and Jolivet, 2002; Jolivet, Verma and Mille, 2003; Verma and Jolivet, 2004); hence they have been regarded as monophyl-etic. Bohumiljania of New Caledonia and Hornius of South America are particularly close. Really, this is a good example of the Gondwanian distribution, as for many other beetles which are common between Australia and Patagonia, at least at the genus level due to close association between the Australian region and S. America through the Antarctica in the Gondwanian history. The recent discovery of an Australian species of Sphaenognathus in the mountains of Queensland, by Monteith (Moore and Monteith, 2004), is an event of considerable zoogeographical importance, this lucanid being known outside only from the neotropical region. It is another Gondwanian relic.

As has been pointed out earlier, it is known that till late Cretaceous (about 75 million years ago) Australia and South America were almost connected through Antarctica, while Africa, Madagascar, with the Seychelles plateau, and India had drifted away. Presence of the primitive eumolpines only in the Australian region and in South America and their absence in other land masses of the Gondwanian origin may be explained in the same way as for the marsupials, that is due to long isolation of South America and Australia from other land masses and consequent better survival of primitive forms in absence of competition with higher and more successful forms. That Megamerus is also present in Madagascar, in addition to South America and Australia, is another pointer to Gondwanian origin of the former.

For reasons not clear to us leaf beetles of the subfamily Chrysomelinae, Paropsini are confined to Australia, New Guinea and some other neighbouring islands. Perhaps these chrysomelines evolved after separation of the Australian plate from other land masses of the Gondwanian origin, and then, finding a variety of vacant niches, they diversified into Paropsini. Cratopus, a colored weevil, originated in Madagascar and the Comores and invaded the Mascareignes, where it diversified explosively. It populated vacant niches on plants and trees, since it has the same habits as chrysomelid beetles, feeding openly on leaves.

Jeannel (1942) was an ardent supporter of the Gondwana Hypothesis. But he has been criticized by those who have studied the Madagascar fauna. That the fauna of this large island closely resembles the African fauna and shares not much with the fauna of other land masses of the Gondwanian origin is mainly because of its proximity with Africa. As Mahe (1972) has pointed out, there was a large peninsula shaped of relatively shallow sea between the western coast of Madagascar and Africa, and also that during the Pleistocene, when most water was locked up as ice, perhaps there was a chain of islands between Africa and Madagascar. Such stepping stones were the way, people believe, Galapagos islands were popuIated with big tortoises and endemic apterous insects. In spite of the comparatively recent invasion of the African fauna, relics of the Gondwanian origin are still there. Paulian (1972) has pointed out some such forms in the Madagascar fauna, for example: (a) ".. .Orphninae which have produced some very well differentiated Malagasy forms whose equivalent is found only in the Congo and the islands of the Gulf of Guinea or in South America.", (b) ".the Blepharocerid Edwardsininae are now found only in South America, Australia and Madagascar; their Malagasay representative, Paulianina Alex, is highly specialized and produces a long series of

Malagasy species...", and (c) "The Plecoptera with the genus Madanemu-ra whose numerous species are related to the South African and New Zealand forms of primitive Neonemouridae...". Endemism is specially pronounced in Madagascar due to its long isolation. There is quite a big quantity of common genera of beetles between South America and Australia.

It is easier to understand the presence of big fossil tortoises in Madagascar than in the Mascareignes, pure volcanic and relatively recenty emerged islands. However the historic presence of a Hippopotamus in the big island of Madagascar remains a dark mystery and no satisfying interpretation has been done until now.

— Fig. 25.2. Gondwana (-80 myr), during the Cretaceous, showing the Southern Gondwana continent, still linked with Patagonia, with Africa, Madagascar and India drifting to North (after Sanmartin, 2002).

— Fig. 25.3. Antarctica (-80 myr). Still — Fig. 25.4. Antarctica (-60 myr). There links with Patagonia and the Australian are still links with Australia, New — Plate persisted (after Sanmartin, 2002). Guinea but separation started from